Electrolytic method of preparing electrical rectifiers



Feb. 6, 1945. P. H. DOWLING ET AL Filed April 11, 1940 imimlulllliimi Caprour Oxide Fig.7.

Cuppaardxzde Elozrolyzically Marker Copper reduced ego 001.

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' 6240mm" Oxide [Zea 1 0 ELECTROLYTIC METHOD OF PREPARING ELECTRICAL REGTIFIERS Modzep Copper I fiedaoedwppen to the element.

Patented Feb. s, 1945 ELECTROLYTIC METHOD OF PREPARING ELECTRICAL RECTIFIERS Philip H. Dowling, Forest Hills, and Herbert L. Taylor, Swissvale, Pa., assignors to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application April 11, 1940, Serial No. 329,132

Our invention relates to electrical rectiflers,

and particularly to electrical rectifiers of the a good metallic contact with the copper oxide layer of a copper oxide rectifier element.

According to one form of our invention we first reduce the outer surface of the oxide layer of a rectifier element of the type described to metallic copper, as by making it the cathode in a suitable reducing solution such as a dilute solution of ammonium hydroxide in water and passing a suitable current through the solution to the element, and we then plate the reduced surface with tin.

According to another form of our inventio we plate the tin directly onto the copper oxide surface of the element by making it the cathode in an acid solution of stannous oxalate and 20 passing a suitable current through the solution Other objects and characteristic features of our invention will appear as the description proceeds.

We shall describe two forms of rectiflers embodying our invention, and'two processes for rectifier manufacture also embodying our invention, and shall then point out the novel features Manufacture of electrical rectifiers.

oxidizing the blank and the subsequent treat- 'ment thereof to remove the excess oxide are well known, and if a detail description of these steps is desired reference may be had to Letters Patent of the United States No. 2,094,642, grantedto Philip H. Dowling, on October 5, 1937, for

Following this treatment the rectifier element will appear as shown in Fig. 1 in which the oxidized blank is designated as a whole by the reference character I, the mother copper thereof by the reference character A, and the coating of cuprous oxide remaining on the inner and outer edges of the blank and on one face A2 of the blank following the removal of the excess oxide by the reference character D. v

The elements are next treated to reduce the outer surface of the cuprous oxide coating of the blanks to metallic copper. This reduction may be accomplished in a variety of ways, but

it is essential that the means employed for reducing the copper should not impair the rectifying properties of the elements, and it is also It is further essential that the surface .of the oxide at the copper-cuprous oxide Junction should not be reduced because if it were reduced the elements would become short circuited and would therefore be rendered use- 1 less for their intended purpose.

One satisfactory method by which the desired reduction may be obtained consists in immersour invention. Fig. 2 is a longitudinal sectional ing the elements in an electrolytic bath comview of an electrolytic bath utilized in the process of manufacture embodying our invention. Fig. 3 is a side view of amasking device illusprising a dilute solution of ammonium hydrox-- ide and passing a current through this bath to the elements from an insoluble anode such as trated in connection with the bath shown in" platinum or carbon, and in Figs. 2 and 3 we Fig. 2 for a purpose which will be made clear in 40 the following specification. Fig. 4 is a vertical sectional view showing a rectifier element as it appears in a further step in the process of man- Similar reference characters refer tosimilar parts in each oi. the several views.

In the practice of our. invention a copper blank is first oxidized as by subjecting it to a temperature of approximately 1860 F. in an oxidizing atmosphere, and the oxidized blank is then treated to remove the excess oxide formed have shown a pair of elements A immersed as the cathode in the electrolytic bath, the bath being contained in a tank 2, and the elements being mounted in a device C which serves both as a means for making electrical contact with the mother copper of the elements, and also as a means for masking the outside and inside edges of the, elements to prevent the copper oxide from being reduced adjacent to the edges where it joins the mother metal. The device Q may assume a variety of forms and, as here shown, comprises a metal washer 3, such for example as brass cemented, as by a latex compound, between two rubber washers and connected to a terminal wire 5. The rubber washduring the oxidizing process. 'The method of ra 4 have substantially the same thickness as the elements A in the stage in which they are shown in Fig. 1, and substantially the same inside diameters as the, outside diameters of the elements, and the elements are'disposed within the openings in these washers, with their unoxidized faces in contact with the metal washer 3. The outside edges of the elements are masked by means of a pair of rubber washers 6 having inside diameters that are slightly smaller than the outside diameters of the elements, and which washers are clamped against the outer edges of the oxidized faces of the elements in concentric relation thereto b means of insulating washers 1 and bolts 8, whereby portions of the washers 6 overlap the oxidized faces of the elements adjacent their outer edges and serve to substantially seal theoxide surfaces against the action of the electrolyte fora small annular area inside of the adjacent copper-cuprous oxide junction. provided at their upper edges with one or more vents 6a to allow the escape of gases. The inside edges of the elements are masked by means of a pair of rubber washers '9 having outside diameters which are slightly larger than the diameters of the holes in the elements, and which The washers 6 are preferably each washers are clamped against the inside edges of the elements in concentric relation to the elements by means of a pair of insulating clamping members Ill and II in such manner as to substantially seal the copper oxide surfaces against the action of the electrolyte for a small annular area adjacent to the inner copper-cuprous oxide junction.

An anode is preferably provided for each element, and as shown in Fig. 2, each of these anodes consists of an insoluble electrode l3, such as platinum, to which external connections are made by a. conductor H. To accomplish the desired reduction, current is of course passed from the electrodes l3 to the rectifier elements,

' and it-has been found that with an ammoniacal solution consisting of .2%' by volume of concentrated. NHaOH (28%NH3) is distilled water at room temperature excellent results may be obtained with a current density of about 12 amperes per. square foot maintained for a period of from 3 to 8 minutes with mechanical agitation of the bath, the reduced copper under these conditions being bright, fine grained, extremely adherent, and disposed as a continuous film over the entire exposed face of the element.

The amount of the reduction which takes place depends upon the temperature of the reducing solution, the length of time during which the reduction is continued, and the magnitude of the reducing current, and while the values given above for these items appear to be the preferred values, satisfactory results can be obtained with other values. In general, the rate of reduction is increased with increased temperatures, but the temperature of the solution should be maintained below 30 0., preferably between C. and C. If the temperature becomes too high, the reduction may be carried into the oxide too far, and a short circuited element will result. The ammoniacal solution specified has a relatively high resistance which necessitates arelatively high voltage, and tends to cause heating of the solution by the reducing current: Lower concentrations 01' the ammonium hydroxide 'can be used, but the voltage required to effect the reduction increases as the concentration decreases. To facilitate temperature control it is desirable that the reducing solution be of a relatively low resistance, and this result may be obtained either by using a higher concentration of NH4OH, or by adding a small quantity of a soluble neutral salt to the above. specified solution. For example, if .05%

y volume of a saturated (25 C.) solution of (NH4)2SO4 is added to the above solution, the voltage required to accomplish the reduction will be decreased Excellent results can also be obtained by using a, very dilute ammoniacal solution of ammonium carbonate. This latter solution can be made by passing gaseous CO2 through a solution of NH4OH in distilled water. The concentration of the NH40H should be very low, i. e., in the neighborhood of .2% by volume of the usual concentrated NH4OH (28.5% NHs, sp. grav.=.9) or about .05% NH? by weight.

The concentration of the CO2 in thesolution is not critical, but if the CO2 is addeduntil the 'specific resistance of the solution at 25 C. is in the vicinity of 850 ohm-cm, excellent results are obtained. When this latter solution is used, there is a tendency to produce a streaked copper "surface unless the solution is agitated. This may be accomplished by moving slightly the element whose copper surface is being reduced. For a 1 element, a motion of A" parallel to the face-of the element at the rate of one or two cycles per second is sufficient.

Increasing the amount of the reduction decreases the forward resistance of the elements,

but if the reduction is carried on for too great a length of time, there is danger of short circuiting the rectifier element due to the copper formed by the reducton penetratng to the mother copper. The allowable amount of the reduction depends upon the original thickness of the oxide which, in turn,.depends principally upon the temperature and length of time of the original oxidation.

If the reduction of the copper oxide surface of the elements is prolonged to decrease the forward resistance of the elements, it is necessary in order to avoid danger of short circuiting' the elements that the reduction should be uniform over the entire portion of the surface which is being reduced. As there is naturally a tendency for the current to concentrate at the edges of the elements, the elements should be shielded, or some device should be employed which will reduce the concentration at the edges. O1ne convenient means for accomplishing the necessary shielding consists in making the washers -1 which clamp the masking washers 6 against the outer edges of the elements with an inside diameter of such size that the inner edges of the washers 1 will project past the inner edges of the washer 6 for some distance in the manner shown in Fig. 2.

Upon removing the elements from the electrolytic reduction bath, the elements are thoroughly rinsed in several rinse waters, the last rinse water preferably being distilled water, and

the elements are then preferably, although not necessarily, subjected to a vigorous blast of compressed air. The air blast appears to have the effect of blowing out some of the gas and liquid which collects in the pores of the reduced copper,

ing solution in the same manner that they were.

made the cathode in the ammonium hydroxide bath during the preceding step.

The particular tin p ating solution employed is very important, and We have found that solutions which are alkaline in character, such as the standard commercial stannate solutions are entirely unsuited for plating tin on copper oxide or reduced copper oxide surfaces. It is only in acid solutions that satisfactory coats of tin can be obtained and even in these solutions the acidity must not be too high. In fact, we have found that best results are obtained in solutions having a pH value of between 2, and 4, preferably nearer the latter. Outside of this range difficulty is not only had in producing satisfactory contacts, but the tin may not deposit on the oxide or reduced copper surface at all. The well-known difficulty of keeping tin salts in solution in this pH range greatly limits the tin salts that can be used as a satisfactory electrolyte. One solution whlch can be satisfactorily used for lproducing good coatings of tin on copper oxide comprises If desired, 30 grams per liter of dextrine can be used in place of the gelatine. The solution may be utilized at room temperature.

The proper plating voltage should preferably be applied to the elements before they are immersed in the plating solution. A plating current of 17 amperes per square foot maintained for about three minutes produces a satisfactory coat of tin'on a reduced copper surface which has been formed on the copper oxide layer by the preferred reducing. process described above. Agitation of the tin plating solution appears to be desirable.

After the desired layer of tin plating has been obtained, the rectifier elements are thoroughly rinsed in water and dried in an air blast.

An element which has been tin plated in the manner just described appears as shown in Fig. 5. The tin layer is very bright, white and extremely adherent. The electrical resistance between the tin and the oxide layer is likely to be high immediately after plating, but after a few hours at room temperature the resistance will drop to its normal value. This action may be accelerated, if desired, by a bake at 80 C. for one hour or equivalent treatment.

If desired, the preliminary reduction of the copper oxide to metallic copper may be elimihated and the tin may be plated directly onto the cuprous oxide surface of the element. This plating may be accomplished in the oxalate solution described above, and fair results may be obtained with a plating current of 12 amperes per square foot maintained for about three minutes. The effect of humid air on contacts so produced is however much more pronounced than is the case when the outer surface of the oxide is first reduced to metallic copper and this reduced surface is then plated with tin, as described hereinbefore.

Although we have herein shown and described only two forms of electrical rectifiers embodying our invention, and two processes for manufacturing said rectifiers, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. The process for making contact with the copper oxide layer of a copper oxide rectifier element which consists in making the element the cathode in a tin plating solution comprising stannous oxalate-50 grams per liter, ammonium oxalategrams per liter, oxalic acid- 15 grams per liter, gelatine2.5 grams per liter, said solution having its pH value maintained between 2 and 4, and passing a current through said solution sufficient to deposit tin on the outer surface of the oxide.

2. The process for making contact with the copper oxide layer of a copper oxide rectifier element which consists in reducing the outer surface of the oxide layer to metallic copper, [making the element the cathode in a tin plating solution comprising stannous oxalate-50 grams per liter, ammonium oxalate60 grams per liter, oxalic acid-15 grams per liter, gelatine-2.5 grams per liter, said solution having its pH value maintained between 2 and 4, and

passinga current through said solution sufficient to deposit tin on the outer surface of the reduced metallic copper.

PHILIP H. DOWLING. HERBERT L. TAYLOR. 

